Solid state, high voltage power converters will be used to convert direct current to alternating current and alternating current to direct current in electrical transmission systems. Those converters will comprise networks of thousands of solid state switching elements, many of which will be connected with all of their terminals at high potentials above ground level. The switching action of those elements must be carefully timed and coordinated to assure that the power converter operates in an efficient and nondestructive manner. The projected cost of insulated pulse transformers, which would be required to trigger solid state switches in a conventional manner, is so large as to make the over-all cost of such an installation prohibitive. It has been proposed, therefore, to employ silicon thyristors which are triggered by radiation, i.e., infrared light, pulses. Radiation may be transmitted from a few sources to large numbers of thyristors in a single installation, via dielectric light pipes which provide electrical isolation.
Light-activated thyristors are, of course, well known to the semiconductor art and are described, for example, in U.S. Pat. No. 3,893,153 to Page et al. for Light Activated Thyristor with High Di/Dt Capability.
A source which is used to trigger light-activated thyristors in power conversion applications should, optimally, supply pulses of infrared radiation between approximately 9000 A and approximately 11,000 A which last for a few microseconds. It is further required that the light source operate at pulse repetition rates of at least 60 pulses per second (pps) and, for economic operation, have a lifetime on the order of 1 year or more: that is, the light source should have a lifetime of at least 2.times.10.sup.9 pulses.
Thyristors have been triggered by means of light-emitting diodes placed in close proximity to the semiconductor chip. However, it is not clear that light-emitting diodes can deliver the high output power levels which are required to compensate for light pipe losses and yet survive for 2.times.10.sup.9 pulses.
Pulsed infrared light output at levels which are sufficient for thyristor triggering may also be obtained from xenon flash lamps, with solid metal electrodes, of the type which are well known to the prior art. However, the lifetime of prior art flash lamps is limited, mainly by the effects of cathode erosion, to approximately 10.sup.6 flashes or less.
The lifetime of prior art rare gas lamps which are commonly utilized in other pulsed service applications is similarly limited. Neodymium-YAG and neodymium glass lasers are typically pumped by flash lamps containing xenon or krypton. These flash lamps are also severely limited in lifetime. At flash energies near the explosion limit lamp lifetime is controlled by envelope failure. At lower flash energies, lamp lifetime is limited by cathode erosion which causes envelope darkening and thus reduces light output.